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Thread: Detached garage load calculation.

  1. #1
    Join Date
    Nov 2007
    Posts
    549

    Detached garage load calculation.

    I have a question regarding figuring a service load calculation for a new residence with a detached garage to be used as a work shop. I have the residence calculation figured but don't know what values to use for the garage. Owner tells me that he will be operating "several 230 volt motors" on various types of wood working equipment. Saw, planer, drill press,etc. I know that most times only a single (possibly 2) motors will be operating at the same time. Would seem foolish to figure full amp load on every possible piece in the garage, even tells me that a welder and compressor will be out there. Any thoughts on how to size the unknown? Thanks.

  2. #2
    Join Date
    Feb 2003
    Location
    Portage, Indiana NEC: 2008
    Posts
    9,654
    Instead of putting the garage on with the house panel, install a 320 meter base with double lugs use one set of lugs to feed the house and the other to go back into the ground over to the garage and feed a 200 amp main breaker panel, this way you only need 3 conductors to the garage as it is still part of the service entrance conductors and is allowed by 230.40 exception 3.

    No disconnect at the house is needed for these conductors as they do not enter the house, and grouping of disconnects is only for each building not each service a common misconception, very common way we do them around here.

    Keep in mind that grounding electrodes at the garage will be required just like any service but you don't seperate the neutrals because it is not being fed by a feeder, you just treat it like any service install you just don't have a meter at the garage.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

  3. #3
    Join Date
    Feb 2009
    Location
    Durango, CO
    Posts
    2,856
    Quote Originally Posted by nizak View Post
    I have a question regarding figuring a service load calculation for a new residence with a detached garage to be used as a work shop. I have the residence calculation figured but don't know what values to use for the garage. Owner tells me that he will be operating "several 230 volt motors" on various types of wood working equipment. Saw, planer, drill press,etc. I know that most times only a single (possibly 2) motors will be operating at the same time. Would seem foolish to figure full amp load on every possible piece in the garage, even tells me that a welder and compressor will be out there. Any thoughts on how to size the unknown? Thanks.
    Feed with 2-2-4-6Al mobile home feeder (at least that is what we call it around here) and a 90A breaker and you will be fine. The owner could fire up anything he wants. I have little doubt you could run the whole thing on #8Cu and a 50A breaker, but the Al is less expensive so why not?

    If you do something like Hurk suggests you could use a 100A main on the garage.
    If Billy Idol or John Denver is on your Ipod go and re-evaluate your life.

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